Motile Cilia on Kidney Proximal Tubular Epithelial Cells Are Associated With Tubular Injury and Interstitial Fibrosis

Tails of nephron ciliated cell development: insights on patterning a functional tissue barrier from the zebrafish

Cilia are hair-like structures found on the surface of nearly all vertebrate cell types where they have central roles in regulating development and orchestrating physiological events. There is growing interest in understanding the mechanisms of ciliogenesis due to the profound consequences that follow from the absence of proper ciliary function, which include diseases that affect the renal, respiratory, reproductive, nervous, visual, and digestive systems, among others. Now, a recent report has discerned new roles for the transcription factor estrogen-related receptor gamma a (esrrγa) in ciliated cell ontogeny within the embryonic zebrafish kidney and other tissues. Further, the team of researchers discovered that genetic ablation of murine homolog ERRγ in adult kidney epithelial cells led to shortened cilia, which precedes cystogenesis. These intriguing findings expand our fundamental understanding of the pathological basis of cilia defects, which is relevant for identifying future therapeutic targets for ciliopathies.

IK is essentially involved in ciliogenesis as an upstream regulator of oral-facial-digital syndrome ciliopathy gene, ofd1

BACKGROUND

The cilia are microtubule-based organelles that protrude from the cell surface. Abnormalities in cilia result in various ciliopathies, including polycystic kidney disease (PKD), Bardet-Biedl syndrome (BBS), and oral-facial-digital syndrome type I (OFD1), which show genetic defects associated with cilia formation. Although an increasing number of human diseases is attributed to ciliary defects, the functions or regulatory mechanisms of several ciliopathy genes remain unclear. Because multi ciliated cells (MCCs) are especially deep in vivo, studying ciliogenesis is challenging. Here, we demonstrate that ik is essential for ciliogenesis in vivo.

RESULTS

In the absence of ik, zebrafish embryos showed various ciliopathy phenotypes, such as body curvature, abnormal otoliths, and cyst formation in the kidney. RNA sequencing analysis revealed that ik positively regulated ofd1 expression required for cilium assembly. In fact, depletion of ik resulted in the downregulation of ofd1 expression with ciliary defects, and these ciliary defects in ik mutants were rescued by restoring ofd1 expression. Interestingly, ik affected ciliogenesis particularly in the proximal tubule but not in the distal tubule in the kidney.

CONCLUSIONS

This study demonstrates the role of ik in ciliogenesis in vivo for the first time. Loss of ik in zebrafish embryos displays various ciliopathy phenotypes with abnormal ciliary morphology in ciliary tissues. Our findings on the ik-ofd1 axis provide new insights into the biological function of ik in clinical ciliopathy studies in humans.